Thin film transistor (TFT), such as In—Ga—Zn—O (IGZO) TFT, is a basic circuit component that can be widely used in various electronic system, and has many advantages, such as high electron mobility, low temperature manufacturing process, high stability, transparency and so on. However, with the present manufacturing process for thin film transistor, due to the increased overlaps between gate and source, and between gate and drain, the gate-source parasitic capacitance increases, so that the overall performance of the thin film transistor becomes bad. Further, there is high requirement of alignment between source contact via and gate, and between drain contact via and gate; low-precision mask focusing can cause the source and drain contact vias to become asymmetric, open circuit or short circuit may even arise, and reliability becomes low.